138 DRAINAGE OF DRAYEOADS AND RAILROADS. 



On well-kept railroads there is practically very little 

 difference in effect, so far as the foundation is concerned, 

 between a rolling load and a stationary load, because the 

 distributed load to be sustained is small in itself, and, owing 

 to the girder action of the rails, even that little is applied 

 gradually. 



In the case of drayroads, there is only one way in which 

 the tendency to displacement of the soil due to traffic can 

 be effectually prevented, and that is by covering the surface 

 with a continuous and sufficiently thick layer of some non- 

 absorbent, hard, and tenacious substance, which, in the absence 

 of any better material, generally consists of timber, stone, or 

 gravel. This covering serves three purposes : — 



1st — It sheds the rain-water, and keeps the foundation 



comparatively dry ; 

 2nd — It serves to distribute the rolling load over a 



greater area of subsoil ; 

 3rd — It provides a hard and unyielding surface upon 

 which the wheels may revolve with less friction. 



It may be accepted as an axiom that both railroads and 

 ■drayroads constructed in uneven country must be systematically, 

 even if not thoroughly, drained to ensure their existence as 

 roads, quite apart from any consideration of the safe conduct 

 of traffic thereon. But when we come to deal with even 

 ■country the conditions are totally different, and the question 

 becomes much more complex — not only as regards the necessity 

 for drainage, but also the amount that should be provided. 



The question of climate also is a factor that should be 

 taken into account, for in countries subject to severe frost 

 the thorough drainage of railroads at all events becomes a 

 much more pressing matter than in tropical or sub-tropical 

 countries where frost is almost unknown. The road engineer 

 generally solves the difficulty, to his own satisfaction at all 

 events, by forming the road with soil taken from the side 

 ditches, and providing a good thick layer of stone or gravel. 

 In the course of time he finds out the weak places where water 

 will persist in breaking across the road and carrying away the 

 .gravel, etc., and if he is a sensible man and has sufficient money 



